Effect of Internal and External Concentration Polarizations on the Performance of Forward Osmosis Process

Abstract

Forward osmosis (FO) as an osmotically driven membrane process is severely affected by the concentration polarization phenomenon on both sides of the membrane as well as inside the support layer. Though the effect of internal concentration polarization (ICP) in the porous support on the draw solution side is far more pronounced than that of the external concentration polarization (ECP), still the importance of ECP cannot be neglected. The ECP becomes particularly important when the feed flow rate is enhanced to increase the permeation flux by increasing the agitation and turbulence on the membrane surface. To capture the effect of ECP a suitable value of mass transfer coefficient must be determined. In this chapter, an FO mass transport model that accounts for the presence of both ICP and ECP phenomena is first presented on the basis of solution-diffusion model coupled with diffusion-convection. Then, three methods for the estimation of mass transfer coefficient based on empirical Sherwood (Sh) number correlations, pressure-driven reverse osmosis (RO), and osmosis-driven pressure retarded osmosis (PRO) are proposed. Finally, a methodology for the prediction of water flux through FO membranes using the theoretical model and calculated/measured parameters (hydraulic permeability, salt resistivity of the support layer, and mass transfer coefficient) is presented.

Fabrication of fouling resistant Ti3C2Tx (MXene)/cellulose acetate nanocomposite membrane for forward osmosis application

“…DECP in PRO mode. (Bhinder et al, 2018) Generally, the ECP phenomenon in both types decreases the effective osmotic driving force in other words reduces the water flux. This unfavorable impact of the ECP can be reduced by increasing circulating flow rate or mixing that minimize the boundary layer thickness.…”

“…CECP in FO mode. (Bhinder, Shabani, & Sadrzadeh, 2018) Dilutive external concentration polarization (DECP) is developed due to the reduction of draw solution concentration at the AL-DS interaction point which is a result of DS dilution as the water diffused from feed soution to draw solution. This reduce ∆ to be less than the ∆ as in the FO mode; however, the ∆ in this mode is greater than that in FO mode that is depicted in Figure 4.…”

Shale Oil and Gas Produced Water Treatment: Opportunities and Barriers for Forward Osmosis

“…In FO membranes, CP occurs because of a concentration gradient between FS and DS through an asymmetric FO membrane [15,18,37]. Furthermore, CP can be broken into internal concentration polarization The ECP phenomenon occurs around the surface of the FO membrane's active layer and is caused by a difference in the membrane surface's concentration and the concentration of the bulk solution [38]. Moreover, ECP can be defined further by two types, concentrative and dilutive.…”

“…The ICP phenomenon occurs within the FO membrane's support layer and is when solutes are unable to penetrate the dense active layer, which causes the CP within the porous support layer [15,38]. ICP can also be defined further by two types, concentrative and dilutive.…”

“…An ideal FO membrane consists of an active layer that high water permissibility and a support layer that minimizes CP, both internal and external, and thus enhances [38].…”